Documentation: Add small fixes

* Remove empty security.md
* Remove second H1 header from lib/index.md
* Move two documents in appropriate subfolders
* Fix file path
* Drop document overview

Change-Id: I0e9df6203e82003c01b84967ea6bd779d7583fef
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32340
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Martin Roth <martinroth@google.com>

Documentation/lib/abi-data-consumption.md

@@ -0,0 +1,25 @@
+# ABI data consumption
+
+This text describes the ABI coreboot presents to downstream users. Such
+users are payloads and/or operating systems. Therefore, this text serves
+at what can be relied on for downstream consumption. Anything not explicitly
+listed as consumable is subject to change without notice.
+
+## Background and Usage
+
+coreboot passes information to downstream users using coreboot tables. These
+table definitions can be found in
+`./src/commonlib/include/commonlib/coreboot_tables.h` and
+`./payloads/libpayload/include/coreboot_tables.h` respectively within coreboot
+and libpayload. One of the most vital and important pieces of information
+found within these tables is the memory map of the system indicating
+available and reserved memory.
+
+In 2009 cbmem was added to coreboot. The "CBMEM high table memory manager"
+serves a way for coreboot to bookkeep its own internal data. While some
+of this data may be exposed through the coreboot tables the data structures
+used to manage the data within the cbmem area is subject to change.
+
+Provided the above, if one needs a piece of data exposed to the OS
+or payload it should reside within the coreboot tables. If it isn't there
+then a code change will be required to add it to the coreboot tables.